Second order gradient directional microphone



May 26, 1953 H. F. OLSON EIAL SECOND ORDER GRADIENT DIRECTIONALMICROPHONE Filed Oct. 29A, 1949 3nnentor$ ahw Pegsron (Ittomeg PatentedMay 26, 1953 SECOND ORDER GRADIENT DIRECTIONAL MICROPHONE Harry F.Olson, Princeton, and John Preston,

Metedeconk, N. J assignors to Radio Corporation of America, acorporation of Delaware Application October 29, 1949, Serial No. 124,461

8 Claims.

The present invention relates to sound translating apparatus, and moreparticularly to a second order, pressure gradient responsive microphonewith a high order of directivity over a wide range of useful soundfrequencies.

Directional microphones are universally employed to discriminate againstundesirable sounds normally picked up by the acoustical system of soundtranslating apparatus. It has been established that a directional soundcollecting system with a directiviy pattern which does not varymaterially with the frequency is the most desirable, being particularlyuseful in recording sound motion pictures, television, orchestras,radio, stage productions and in many sound reenforcing applications.

One of the most important applications is that of sound pickup intelevision. The action in television covers a long period of time and arelatively large range, with rapid changes in pickup points. Inaddition, it is becoming increasingly desirable that the microphone bekept out of the picture at all times. In the sound pickup systems in usetoday, the microphone is usually mounted at the end of a boom which canbe moved around by the operator to cover the action and at the same timekeep the microphone out of the picture. This system is quite cumbersome,and difficulty is experienced at times in covering the action. If amicrophone with a high order of directivity is employed, it is possibleto use a number of these microphones arranged in fixed, spaced positionsto cover the entire area of action. As the action changes from one areato another, the appropriate microphone may be brought into action, as byfading between microphones through a control device or monitoringconsole. It will be seen readily that, under this improved soundcollecting system, the action can be covered in a more satisfactorymanner than by means of a boom, particularly when the action is complexand shifts rapidly from one part of the stage to another. In additionthereto, there is another outstanding advantage for this method of soundpickup, namely, that the sound is picked up from the front of the stagerather than overhead as in the case of boom microphone pickup. The netresult is that better illusion is obtained because the sound correspondsto the picture.

It is, of course, well known that directional sound collecting systemsmay be divided into two classes, namely, wave and gradient types. In thecase of gradient systems, which depend upon differences in pressure, twomajor problems are encountered, namely, (1) similarity of frequencyresponse of the units to obtain proper balance, and (2) adequatesensitivity. We have been successful in overcoming these two problems byour present invention and have developed a second order pressuregradient microphone with a high order of directivity and sensitivity,and which is particularly adapted for use in a sound collecting systemsuch as that outlined above.

A second order pressure gradient responsive microphone with aunidirectional directivity pattern may be obtained from a combination oftwo first order pressure gradient responsive microphones and a suitabledelay network, or from the combination of two unidirectional microphoneseach consisting of a first order, pressure gradient responsivemicrophone and a 'zero order, pressure radient responsive microphone.

The theory of operation of both of these systems is explained in OlsonPatent No. 2,301,744. Of the two systems, the latter system has beenfound more suitable, particularly when a broad frequency range isdesired.

It is Well known that the upper limit of the useful frequency range of asecond order, pressure gradient responsive microphone of the typeemploying two unidirectional microphones, such, for example, as themicrophone shown and described in Olson Patent No. 2,301,638, isdetermined by the distance between the units. This upper frequency limitis given by c f v 3 1 where fc=upper frequency limit in cycles persecond, c=velocity of sound, in centimeters per second,.

and d=distance between the units in centimeters.

The voltage output of a gradient microphone of the type mentioned above,in the low frequency range, that is, in the range for which d is givenby d=distance between the units, in centimeters, and

i=wavelength, in centimeters.

If the upper limit is 10,000 cycles per second,

the distance between the units will be relatively small, as given byEquation 1. Under these conditions, Equation 2 shows that the voltageoutput in the low frequency range will be low. There are twoalternatives, namely, (1) to use two systems of the type employing twounidirectional microphones, as mentioned above, one covering the lowfrequency range, with a relatively large distance between the units, andone covering the high frequency range with a relatively small distancebetween the units, or (2) to use a pressure gradient system of the typementioned above for the low frequency range and a wave type system fordirectivity in the high frequency range. We have found the latter typeof microphone to be the preferred type from the standpoint of overallsize and weight, and since it requires fewer elements, it is easier andless costly to produce.

The primary object of our present invention, therefore, is to provide asecond order pressure gradient responsive microphone with a high orderof directivity over a wide range of useful sound frequencies.

Another object of our present invention is to extend the useful range ofsound frequencies of a second order, pressure gradient responsivemicrophone while at the same time preserving a high order of directivityand sensitivity.

It is also an object of our present invention to provide an improvedsecond order pressure gradient responsive microphone which has lineardimensions which are relatively small, and one which is simple inconstruction, yet highly efficient in use.

In accordance with our present invention, our improved second order,pressure gradient responsive microphone comprises a pair ofunidirectional microphone units disposed in tandem along a common axisand with the vibratile elements thereof mounted in predetermined,spaced-apart relation and facing in the same direction so that theirdirectional axes of maximum response are substantially in common. Anacoustic resistance in the form of a labyrinth pipe structure is mountedbehind each vibratile element with an opening in each pipe behind thevibratile element of a sizeto impart a unidirectional characteristic toeach microphone unit. The labyrinth pipe structurealso serves as aframework for supporting the respective magnetic structures with theirassociated vibratile elements, as well as the output transformer andelectrical crossover network, therebetween. The two microphone units areeffectively connected so that their signal output voltages are inopposite phase relation and function as a second order, pressuregradient responsive microphone below substantially a predetermined soundfrequency. A capacitor is connected in circuit with one of themicrophone units for cancelling the signal output voltage thereof abovethis, predetermined sound frequency so that the microphone assemblyfunctions as a first order pressure gradient microphone. Thus, themicrophone assembly operates as a first order, pressure gradientresponsive microphone in'the higher range of sound frequencies, and as asecond order, pressure gradient responsive microphone in the lower rangeof sound frequencies.

The novel features characteristic of our invention, both as toorganization and method of operation, as well as additional objects andadvantages thereof, will be understood better from the followingdetailed description, when read in connection with the accompanyingdrawing in I which Figure 1 is a perspective view, as seen from oneside, of a second order, pressure gradient responsive microphone inaccordance with one form of our present invention, the protective screenenclosure having been removed,

Figure 2 is a perspective view of the microphone shown in Fig. 1, asseen from the front,

Figure 3 is an enlarged side view of the labyrinth connector mountedbehind the ribbon element of one of the microphone translating units andemployed to couple the vibratory element of this unit to the dampedlabyrinth pipe,

Figure 4 is an end view of the labyrinth connector shown in Fig. 3,

Figure 5 is a curve showing the directional characteristic of either ofthe unidirectional microphone units shown in Fig. 1,

Figure 6 is a curve showing the combined directional characteristics ofthe two unidirectional microphone units shown in Fig. 1, and

Figure '7 is a diagrammatic view showing the microphone output circuitarrangement.

Referring more particularly to the drawing, wherein similar referencecharacters designate corresponding parts throughout, there is shown asecond order, pressure gradient responsive microphone I comprising twounidirectional microphone units 3, 5 of the dynamic type each of whichis similar to a unidirectional microphone of the kind shown anddescribed in the above mentioned Olson Patent No. 2,301,638.

Each of the unidirectional microphone units 3, 5 comprises a magneticstructure 1 having an air gap 8 Within which a conductive ribbon orother vibratile element 9 is disposed for vibratory movement in responseto sound wave energy impinging on opposite sides thereof. The ribbonelement 9 is exposed on one side to incident sound waves, and the otherside of the ribbon S is enclosed and terminates in an accousticalresistance in the form of a labyrinth structure ,or closed pipe i Iwhich is filled with suitable damping material. The labyrinth structureII, for convenience of manufacture and assembly, comprises two parts,namely, an elongated, folded pipe 13, and a labyrinth connector 14 whichis disposed directly behind and encloses the ribbon 9. An opening [5 isprovided in the labyrinth connector 14 directly behind the ribbon 9 toprovide acoustic inertance to sound waves approaching the ribbon 9through the opening, and a screen ll of fine mesh is mounted over theopening to provide an acoustic resistance for the opening I5, in amanner more fully disclosed, for example, in our copending applicationSerial No. 687,419, filed July 31, 1946. By suitable choice ofconstants, each microphone unit is constructed to produce a directionalresponse characteristic having substantially the same cardioid pattern,such as that illustrated by the curve 18 in Fig. 5 of the drawing.

The two, folded pipes l'3 are spaced from each other laterally, asclearly shown in Figs. 1 and v2, the microphone units 3 and 5 beingmounted therebetween in spaced-apart relation and in proximity to theends of the two labyrinth structures.

The effective length of each of-theclosed pipes 13, whichare required tomaintain proper phase relations in the low frequency range, is aboutinches. Therefore, in order to provide a compact unit or assembly, theclosed pipes l3 are folded in, the manner shown in Figs. 1 and '2.

While the respective folds IQ of the pipes l3 can be disposed in anysuitable manner, they should be spaced apart a sufiicient distance toprovide a completely open framework to provide accessibility for soundwaves to opposite sides of the ribbon elements 9 and to reduceobjectionable resonances which may occur between the labyrinthstructures. The labyrinth structures or frameworks Ii are appropriatelyspaced apart so that the magnetic structures 1 of the units 3, as wellas the transformers 2|, the condenser 25 and other elements of theelectrical crossover network can be mounted or supported therebetween.

The two microphone units 3, 5 should be very smooth and free ofirregularities in response, and the sensitivity of the two units must bethe same within a fraction of a decibel over the entire operatingfrequency range in order to function efiiciently. If these conditionsare not maintained in the gradient frequency range a high order ofcancellation will not be obtained. In order to provide these conditions,special techniques should be followed. For example, the distance betweenthe ribbon elements 9 of the two microphone units 3, 5 should beapproximately one-half the wave length of the highest frequency to whichthat one of the microphone units 3, 5 which is cut off at substantiallya suitable mild-frequency, as hereinafter set forth, is responsive andsmall compared to the wave length at the lowest frequency to which thesystem is responsive. The outputs of the two microphone units 3, 5should be connected in series and in phase opposition so that, together,the response corresponds to the pressure gradient of the pressuregradients in the range below said mid-frequency. As shown in theelectrical circuit of the microphone in Fig. 7 of the drawing, acapacitor 25, which has a value suflicient to cut off the signal outputof one of the microphone units 3, 5 at approximately the upper end ofthe operating range of the system as a second order pressure gradientresponsive microphone (that is, at substantially the aforementionedmid-frequency), is connected in circuit with the output of themicrophone unit to be cut off so that the remaining microphone unit isalone responsive to sound waves above substantially that mid-frequencycut-off frequency. Thus, according to one second order, pressuregradient microphone, which was designed and constructed similar to thatillustrated in Figs. 1 and 2, the two microphone units 3, 5 weredisposed 12 inches apart with the exposed sides of the ribbon elements 9facing in the same frontal direction. The configuration of therespective parts of the magnetic structures i and the elements of thecompound acoustical networks which terminate the ribbons were chosen sothat diffraction and the acoustical network cooperate to form adirectional system. The upper end of the operating range of this system,as a second order, pressure gradient responsive microphone isapproximately 1000 cycles. In the overlap frequency band from 1000cycles to 2000 cycles, the system operates as a combination gradient anddiffraction system, and in the frequency range above 2000 cycles, theone unit, preferably the front unit, operates alone.

From the foregoing description, it will be apparent that we haveprovided an improved, compact, highly directional second order gradientmicrophone employing two unidirectional microphone units so related asto provide a directional pattern given by the expression (1+cos 0) cos0, where 0 is the angle between the direction of the incident sound andthe major axis of the microphone, as represented by Figure 6. The pickupdistance of the microphone described herein is considerably greater thanthat of the unidirectional microphone with a cardioid characteristic forthe same reproduced reverberation and ambient noise.

While we have described and illustrated but a single modification of oursecond order pressure gradient responsive microphone, it will, of

course, be obvious to those persons skilled in the art that variouschanges and modifications are possible within the spirit of ourinvention. Therefore, we desire that the particular form of ourinvention described herein shall be considered as illustrative and notas limiting.

What is claimed is:

l. A sound translating device comprising a pair of microphone units eachof which comprises (1) a magnetic structure having an air gap with amagnetic field therein, (2) a conductive element mounted in said fieldfor vibration in response to acoustical waves, and (3) means closing oneside of said conductor providing acoustic resistance and inertance tosound waves approaching said conductor from said one side, and (4) apair of labyrinth pipe structures, each of said pipe structures beingassociated with a separate one of said microphone units, said means forclosing one side of each of said conductors constituting coupling meansbetween said conductors and their respectively associated labyrinth pipestructures, said conductors being disposed in spaced apart relation, andmeans for eifectively connecting the signal output voltages of saidmicrophone units in opposite phase relation, said last mentioned meansincluding means connected in circuit with one of said microphone unitsfor cancelling the signal output voltage thereof above a predeterminedfrequency.

2. A sound translating device according to claim 1, wherein saidconductors are mounted substantially parallel and in tandem along acommon axis with the side opposite said one side facing in the samefrontal direction.

3. A sound translating device according to claim 2, wherein saidmicrophone units have substantially the same sensitivity, and whereinsaid conductors are spaced apart a distance of the order of one-halfwave length of the highest frequency to which said microphone unit isresponsive and small compared to the wave length at the lowest frequencyto which said one microphone unit is responsive.

4. A sound translating device according to claim 3, wherein saidlabyrinth pipe structures comprise separate, folded frameworks, whereinsaid magnetic structures of each of said units are mounted between saidframeworks, and wherein the respective folds of each of said frameworksare spaced apart, thereby to provide an open framework.

5. A sound translating device comprising a pair of relatively elongated,reversely folded labyrinth structures, said labyrinth structures beingspaced from each other laterally, and a pair of microphone units mountedbetween said labyrinth structures in spaced relation to each other, eachof said microphone units including means for providing said units with aunidirectional response characteristic and comprising a magneticstructure having an air gap with amagnetic field therein, .a conductiveelement mounted in said field responsive to sound waves impingingthereon, and coupling means mounted behind each of said conductiveelements for closing one side thereof, said coupling means beingconnected respectively to separate ones of said labyrinth structures.

6. A sound translating device comprising a pair of relatively elongated,reversely folded labyrinth structures, said labyrinth structures beingspaced from each other laterally, and a pair of microphone units mountedbetween said labyrinth structures in proximity to the ends thereof andin spaced relation to each other, each of said microphone unitsincluding means for providing said units with a unidirectional responsecharacteristic and comprising a magnetic structure having an air gapwith a magnetic field therein, a conductive element mounted in saidfield responsive to sound waves impinging thereon, and coupling meansmounted behind each of said conductive elements for closing one sidethereof, said couplingmeans being connected respectively to separateones of said labyrinth structures.

7. A sound translating device comprisinga pair of relatively elongated,reversely folded labyrinth structures, said labyrinth structuresbeingspaced from each other laterally,and'a pair of microphone units mountedbetween said labyrinth structures in spaced relation to each other, eachof said microphone units including means for providing said units with aunidirectional response characteristic and comprising a magneticstructure having an ,airgap witha magnetic 'iield therein, a conductiveelement mounted in said field responsive to sound wavesimpingingthereon, and coupling means mounted behind each of said conductiveelements for closing one side thereof, said coupling means beingconnected respectively to separate ones of said labyrinth structures,each of said coupling means having an opening therein disposed directlybehind the conductive element associated therewith.

8. A sound translating device comprising a pair of relatively elongated,reversely folded labyrinth structures, said labyrinth structures beingspaced from each other transversely, and a pair of microphone unitsmounted between said labyrinth structures in proximity to the endsthereof and in spaced relation to each other, each of said microphoneunits including means for providing said units with a unidirectionalresponse characteristic and comprising a magnetic structure having anair gap with a magnetic field therein, a conductive element mounted insaid field responsive to sound waves impinging thereon, coupling meansmounted behind each of said conductive elements for closing one sidethereof, said coupling means being connected respectively to separateones of said labyrinth structures, each of said coupling means having anopening therein disposed directly behind the conductive elementassociated therewith, and acoustic resistance means disposed over saidopenings.

HARRY F. OLSON. JOHN PRESTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,237,298 Baumzweizer Apr. 8, 1941 2,301,638 Olson Nov. 10,1942 2,301,744 Olson Nov. 10, 1942 2,305,599 Bauer Dec. 22, 1942

